Upload
others
View
35
Download
1
Embed Size (px)
Citation preview
Lecture 3
Remote Terminal Units
(RTU)
3/28/2021 8:27 AM 1
3/28/2021 8:27 AM 2
Building blocks of SCADA systems
• The SCADA system has four components, the first being the remote terminal unit (RTU) or
data concentrator, which is the link of the control system to the field, for acquiring the data
from the field devices and passing on the control commands from the control station to the
field devices.
• Modern-day SCADA systems are incomplete without the data concentrators and intelligent
electronic devices (IEDs) which are replacing the conventional RTUs with their hardwired
input and output (I/O) points.
Components of SCADA systems
3/28/2021 8:27 AM 3
• The second component is the communication system that carries the monitored data from
the RTU to the control center and the control commands from the master station to the RTU
or data concentrator to be conveyed to the field.
• The communication system is of great significance in SCADA generally and in power
automation specifically, as the power system field is widely distributed over the landscape,
and critical information that is time bound is to be communicated to the master station and
control decisions to the field.
• The third component of the SCADA system is the master station where the operator
monitors the system and makes control decisions to be conveyed to the field.
• The fourth component is the user interface (UI) also referred to as the human-machine
interface (HMI) which is the interaction between the operator and the machine.
• All automation systems essentially have these four components, in varied proportions
depending on the process requirements. Power system SCADA systems are focused on the
master stations and HMI is of great significance, whereas process automation is focused on
controllers, and master station and the HMI has less significance.
3/28/2021 8:27 AM 4
Remote terminal unit (RTU)
• The RTU is the eyes, ears, and hands of the SCADA system.
• In older days, RTU was a slave of the master station, but now RTUs are equipped with
internal computational and optimization facilities.
• RTU collects data from the field devices, processes the data, and sends the data to the
master station through the communication system to assist the monitoring of the power
system as “eyes” and “ears” of the master station.
• At the same time, the RTU receives control commands from the master station and
transmits these commands to the field devices, thus justifying the comparison to the
“hands” of the master station.
3/28/2021 8:27 AM 5
The new RTU systems had the following advantages:1. Modular system development capability2. Largely preprogrammed user interface system that is easy to adapt to the
individual process3. Preprogrammed menu-driven software (final programming using a few buttons
on keyboard)4. Wide selection of control algorithms with preprogrammed menu
5. Data highway with transmission and communication capabilities between separate units ,
wideband, redundancy
6. Relatively easy communication with the control room for supervisory control
7. Extensive diagnostic scheme and devices for easy maintenance and replacement of circuit
board (card level)
8. Redundancy at any level to improve the reliability
9. Industry standard communication protocols (IEEE 1815 or DNP3, IEC 60870-5-101 and
103)
3/28/2021 8:27 AM 6
Components of RTU
RTU has the following major components to monitor and control the field devices:
1. Communication Subsystem: Is the interface between the SCADA communication network
and the RTU internal logic. This subsystem receives messages from the master, interprets
the messages, initiates actions within the RTU which in turn initiates some action in the
field. RTU also sends an appropriate message to the master station on the completion of the
task. It also collects data from the field, and processes and conveys relevant data to the
master station. RTU may report to a single master or multiple masters.
2. Logic Subsystem: The logic subsystem consists of the main processor and database and
handles all major processing, time keeping, and control sensing. The logic subsystem also
handles the analog-to- digital conversions and computational optimization, in most of the
cases.
3/28/2021 8:27 AM 7
3. Termination Subsystem: The termination subsystem provides the interface between RTU
and external equipment such as the communication lines, primary source, and substation
devices. RTU logic needs to be protected from the harsh environment of the substation.
4. Power Supply Subsystem: The power supply converts primary power, usually
from the substation battery, to the supply requirements of the other RTU
subsystems.
5. Test/HMI Subsystem: This subsystem covers a variety of components, built-in
hardware/firmware tests, and visual indicators, within the RTU, and built-in or
portable test/maintenance panels or displays.
3/28/2021 8:27 AM 8
Components of RTUTypical RTU in a substation
3/28/2021 8:27 AM 9
The RTU communication subsystem handles the following functions:
1) Communication protocols
A large variety of communication protocols exist in the power system, and the RTU
communication system is designed to format and interpret the data in the required
protocol. SCADA communication protocols generally “report by exception” or give
information on the points that have changed since the last scan, to reduce the
communication system load.
2) Message security
The data handled by the SCADA system are critical, and any corruption in the data
can lead to serious consequences. Parity check is the simplest method, where a
single bit is added to the message so that the sum is always odd. Cyclic redundancy
check (CRC) is another error-checking mechanism used, which is more reliable.
3/28/2021 8:27 AM 10
3) Multi-port communication
Modern RTUs have to communicate to the higher SCADA hierarchy to more than one master
station, and at the same time, communicate with peer RTUs and IEDs in a variety of protocols.
The communication subsystem should be designed to handle this capability.
The primary functions of the RTU (Logic subsystem) are time keeping and data acquisition
and processing
1) Time keeping
Sequence-of- events (SOE) logging or time tagging of events is of great significance in power
systems, and the logic subsystem handles this task in the RTU. The RTU also has to perform
many functions on a time basis. The RTU supports time synchronization in addition to time
keeping. Time synchronization of the RTU and the master station takes place through the GPS
receivers which ensure perfect synchronism (1 ms resolution).
3/28/2021 8:27 AM 11
2) Data acquisition and processing
SCADA data come in analog and digital forms. The logic subsystem data acquisition
processing collects and reports both types of data. The analog values are acquired
from transducers connected to the field devices, an example being the current and
voltage values from the transmission lines or transformers. The earlier generation of
RTUs had the analog-to- digital conversion module as part of the RTU, requiring
hardwires to be brought in from the field to the RTU. With the advances in analog-
to- digital conversion techniques and communication networks, field devices are
becoming “intelligent” and can supply digital data directly to a LAN which in turn
can be acquired by the RTU.
3/28/2021 8:27 AM 12
Digital data acquisition is done in four ways:
1. Current status
2. Current status with memory detect number of contact changes since last report
3. Sequence of events (SOE) with time tag
4. Accumulator value a count of the number of contact closures over a period of time
(generally used for energy meter pulse generators)
3/28/2021 8:27 AM 13
Analog data acquisition
• The analog signals, generally a voltage or current that changes over a period of
time and also within a certain range, are generally converted to a 4 to 20 mA
signal by the appropriate transducers. Some utilities also use –1 to +1 mA.
• The analog-to- digital converter circuit converts these signals into binary values
for further transmission or analysis by the RTU.
• The analog signals should be free of noise and electromagnetic interference.
• The 4 to 20 mA current loop signals are generally immune from electrical noise
sources and are the most preferred standard input to the A/D converters.
3/28/2021 8:27 AM 14
Termination subsystem
• The termination subsystem is the interface between the RTU, which is an
electronic device, and the physical world, which is hazardous for the RTU.
• The main function of the termination subsystem is to protect the RTU from the
hostile field environment.
• The substation environment is hostile due to many factors such as surges,
lightning, over voltage and reverse voltage, electrostatic discharge (ESD), and
electromagnetic interference (EMI).
• In the case of process industry, the hazardous environment will include temperature,
humidity, and fumes. The actual provisions of isolation between the RTU logic subsystem
and the field will depend upon individual manufacturer; however, the bottom line is the
RTU will have to be protected from the hazardous environment.
3/28/2021 8:27 AM 15
Digital terminations
• The digital inputs to the RTU are from the various forms of switches in the field and
originate from the electromechanical contacts in switchgear and metering devices.
• Generally the contact sensing is done by an interposing relay powered by a battery, which
provides isolation from the field. Optical isolators are also prevalent, which provide
complete isolation. If the contact input comes from a metering device, then the firmware
contains change detection and pulse accumulation logic.
Analog terminations
• Analog inputs are from the transducers, sensors, transmitters, thermocouples, and resistance
devices, which themselves provide electrical isolation.
• The 4 to 20 mA signals from these devices reach the analog-to- digital conversion unit
through fuses and are grounded at the RTU.
3/28/2021 8:27 AM 16
Testing and human-machine interface (HMI) subsystem
• RTUs located at remote locations are generally unmanned and may not have a
display system or HMI associated with them.
• The panel of the RTU will have a number of LEDs which indicate the status of the
various cards and functionalities of the RTU, which give the personnel an idea
about the status of the RTU.
• The RTU will have its own built-in routines that can test the hardware and
software and give indications on the panel.
• The test results and related information will be passed on to the master.
• Continuous monitoring of the firmware and software of the RTU is done so that
faults and problems can be identified and rectified instantly.
3/28/2021 8:27 AM 17
• With the availability of low-cost LED and LCD displays, RTUs can be fitted with
such display panels that will give the values measured by the RTU to convey
information to the personnel present in the plant floor or substation if necessary.
3/28/2021 8:27 AM 18
Power supplies
• The RTU will have a separate power supply unit, which is powered from a suitable
DC source.
• The most common voltage levels in use are 24 VDC, 48 VDC, and 125 VDC.
Sometimes even 250 VDC may be used in systems.
• RTUs in the transmission and distribution system are located in the substation and
are powered from the substation battery.
• These batteries are floating so that a single fault on either side of the battery to
ground will not cause malfunction, equipment damage, or danger to human
beings.
• Many premises have two voltage levels, say 24 VDC and 48 VDC, the RTU supply
can be easily switched from one to the other, making the system more reliable.
3/28/2021 8:27 AM 19
Advanced RTU functionalities
With the advent of microprocessor technology and with the integrated circuit–based
devices, the RTUs also gained in functionality and versatility. The CPUs became
faster, with more memory and advanced computations possible. In the power
industry, the major advancements have been in the following aspects, as shown in the
figure below.
Advanced RTU functionality of the logic subsystem
3/28/2021 8:27 AM 20
1. Multi-port and multi-protocol operation: RTU at a substation in a transmission
SCADA hierarchy will report to the local master station as well as the regional control
center.
2. Digital interface to other electronic devices: RTU has to be equipped to handle these
digitized data. Generally serial interfaces are used which include RS 232and RS 485 for
such communication.
3. Closed-loop control, computation, and optimization at the RTU level: Complex
computations include using measured values of many parameters to compute a value,
which will be the set point and the resulting control action (controlling the capacitor
banks).
3/28/2021 8:27 AM 21
4) Interface to application functions: The operator can initiate the load reduction, demand
response and other activities using the same system, and the RTU has to establish and
interface with such application programs.
5) Advanced data processing: when a circuit breaker operates, numerous analog point
alarms can be generated (low voltage and low current for all three phases). The important
message to give the system operator is the fact the circuit breaker operated (primary
alarm), but present systems also provide the analog point alarms (secondary alarms), too.
6) Time tagging of analog and digital values for sequence of events recording is
implemented in RTUs. Other functions that can be implemented in the RTU
include distribution automation, volt-ampere reactive (VAR) control and fault
detection, isolation, and service restoration